The present invention relates to a method for producing a magnetic recording medium by applying coating compositions, such as a magnetic solution, a surface protecting solution, and so on, onto a flexible support formed of plastic film, paper, metal or the like.
A roll coating method, a bead coating method, a curtain coating method, an extrusion coating method, and so on, are known coating methods for applying a coating composition onto a flexible support (hereinafter referred to as "web"). In any one of the above-mentioned coating methods, the width of a portion of the web to which the coating composition is applied is narrower than the total width of the web, so that uncoated portions having a width in a range from several millimeters to tens of millimeters remain as edge portions on both sides of the web in the direction of the width of the web.
As seen in extrusion coaters disclosed in Japanese Patent Unexamined Publications Nos. Sho. 57-84771, Sho. 58-104666, Sho. 58-109162, Sho. 58-18069, Sho. 58-18070 and Sho. 60-78664, Japanese Patent Applications Laid-Open Nos. Sho. 59-94657 and Sho. 61-230173, Japanese Patent Examined Publication No. Sho. 49-1459, and so on, when the coating composition is applied onto a continuously traveling web while a coating head is being urged against the web, the tense web is bent at the widthwise edge portions so as to be warped toward the coating head to come into contact with the coating head.
As a result of contact with the coating head, the web is cut so that powder-like shavings are produced. The shavings tend to scatter, resulting in the failure of accurate detection of the recorded signals during reproduction. In addition, shavings deposited onto the contact portion of the coating head change the distance between the web and the coating head so that the thickness of the coating composition becomes uneven at the edge portions, reducing the quality of the coating.
It is known that the same effect as that of the coating head can be provided by a doctor bar or a smoother, as disclosed in Japanese Patent Unexamined Publications Nos. Sho. 51-140703, Sho. 52-84242 and Sho. 54-8646, Japanese Patent Publications Nos. Sho. 48-11336 and Sho. 60-53674.
To solve the above problems, therefore, there has been proposed a method in which a portion between the web and the coating head is lubricated with a liquid by preliminarily applying a solvent onto edge portions of the web where the coating composition is not applied, before applying the coating composition onto the web, for example, as disclosed in Japanese Patent Unexamined Publications Nos. Sho. 61-257268 and Hei. 1-102736.
On the other hand, there has been proposed a method in which portions of the coating head or the like corresponding to the edge portions are notched to increase the distance between the coating head and the web, for example, as disclosed in Japanese Patent Unexamined Publication No. Sho. 61-257263.
Furthermore, there has been proposed a method for use with extrusion coating where a suitable lubricating material, such as polymer, monomer, UV resin, or the like, is applied at least to the lug portion of the web to counter lug cutting in the extrusion coating method, for example as described in Japanese Patent Unexamined Publication No. Hei. 3-116524. That is, for the purpose of preventing the web from cutting, a force urging the extruder blade edge to come into contact with the web at the time of coating of the magnetic coating composition is decreased by applying a material, such as fluorine lubricant, polymer, monomer, UV resin, or the like, at least to the lug portion of the web in the direction of width of the web, and then drying the material. The problem of cutting can be solved very effectively by this method.
According to this method, however, the material, such as polymer or the like, applied to the lug portion remains exposed on a surface of the web after drying. This causes problems in subsequent processing steps.
After coating, the magnetic recording medium is subjected to calendering and then subjected to a heat treatment while being rolled for the dual purpose of hardening the resulting film and removing the thermal stress of the web. Here, the film of polymer or the like applied to the lug portion and exposed on the outside edge portions of the magnetic layer on the web is softened or dissolved so as to adhere to the back of the web. Hence, the surface of coating in the vicinity of the web lug portion is separated from the web surface. Particularly in recent years, thinner webs have been used to provide a thin magnetic recording medium. However, the adhesion of the dissolved polymer or the like to the back of the web creates difficulties in web cutting.